Prior Art
In a Siemens German published patent application No. 2,624,005 dated Dec. 8, 1977, a thin film deposition process called ion plating is described. In the process described in this application, an electron gun evaporation system is combined with a gas discharge system so that the evaporated material from the crucible and the ambient gas within the chamber are ionized in the region between the crucible and the substrate on which the material is being deposited. The arrangement of the apparatus shown in this prior art reference is depicted in FIG. 1 of the drawings. The structure and function of this apparatus is described below.
Vacuum chamber 1 is connected to a vacuum pump (not shown in the Figure). Inside the chamber is a water-cooled crucible 3 containing the material 4 with which the substrate is to be coated. The substrate 5 is attached to a substrate holder 6 which extends through an insulated bushing 7 and is connected to a high voltage source 8. Also located inside the chamber is an electron gun 9, which generates an electron beam 10 which is directed in a deflector system 11 towards the material 4. The deflector system 11 can be either a deflector capacitor or an electromagnet. The crucible 3 and the electron gun 9 may also form a single unit. The crucible 3 is connected to the positive terminal of the voltage source 8, possibly via a bushing 12; it may also be earthed.
For operation, the vessel is evacuated to a pressure of approximately 10-5 torr. Then the electron gun 9 is put into operation and the material 4 in the crucible 3 is heated by means of the electron beam 10. When the high voltage source 8 is switched on, a gas discharge forms inside the vessel 1, the extent of which discharge is indicated by the broken line 13. The evaporated material 4 from the crucible 3 is ionized in the gas discharge in the space between the crucible and the substrate, so that ions 14 of the material 4 bombard the substrate. In order for the gas discharge to be able to form at low pressures of 10-5 torr, an additional device 15 producing ions, in the form of a high-frequency coil, is located in the space between the crucible 3 and the substrate 5, to which coil a high-frequency voltage source 16 is connected during the whole coating process.
The system described in the Siemens reference suffers from the following disadvantages:
(1) The use of a high d.c. voltage on the substrate holder limits the usefulness of the system to conductive substrates and conductive coating materials. Dielectric materials may possibly be coated but at low rates. Conditions at the substrate will change significantly as the insulating coating builds up. Even with conductive coatings there is a possibility that ion bombardment from a high applied voltage may cause atomic level damage to the substrate or film.
(2) The use of an r.f. coil to activate both the evaporant and the background gases also limits the versatility of the system. Activation of background gas and source material cannot be independently controlled and in fact, the low recommended background gas pressure of 10-5 Torr may seriously limit the number of gas ions which can be produced.
(3) The requirement for r.f. inside the chamber significantly complicates the coating process. In addition to costing more than d.c. systems, r.f. tends to cause arcing which can affect the quality of the films produced.
Buhl et al. U.S. Pat. No. 4,448,802, entitled METHOD AND APPARATUS FOR EVAPORATING MATERIAL UNDER VACUUM USING BOTH AN ARC DISCHARGE AND AN ELECTRON BEAM, issued on May 15, 1984 discloses several embodiments of systems which combine an electron gun evaporation system with a high current, low voltage source of electrons. In each of the embodiments shown in the Buhl et al. patent, definite geometric relationships between the high voltage electron beam and the low voltage electron beam are specified. These geometric relationships restrict the relative placement of these components of this system.
FIG. 2 of the drawings illustrates a version of the Buhl et al. system being offered commercially by Balzers Aktiegesellschaft of Lichtenstein as a commercially available coating chamber. In this case the coating chamber 21 is a special chamber with a low voltage, high current source 22 mounted in one side of the chamber, so that the low voltage electron beam 23 will have part of its path to the crucible 24 in common with the path of the high voltage electron beam 25 emanating from the electron gun 26. A substrate rack 27 is not connected in a high voltage circuit, contrasted to the above-mentioned Siemens reference and instead the low voltage arc discharge is formed between the low voltage source 22 and the crucible 24.
In the apparatus shown in FIG. 2 and discussed in the Buhl Pat. No. 4,448,802, the low voltage source 22 is arranged so that the magnetic field guiding the high voltage beam 25 also serves to guide the beam of electrons from the low voltage arc discharge 23. This restricts the geometric arrangement between the electron gun arrangement 26, the crucible 24, and the low voltage electron source 22. Inherent in these geometric restrictions is the requirement that the vacuum chamber 21 be specially designed for the incorporation of the low voltage electron source 22. This makes this technology less readily adaptable to standard vacuum coating chambers and makes it difficult to retrofit existing coating chambers with this new technology approach.
In the Buhl, et al. arrangement a reactive gas is injected directly into the vacuum chamber to be ionized in the low voltage arc discharge within the vacuum chamber.